The present invention relates to a magnetic structural arrangement of an installation for nuclear magnetic resonance tomography with superconducting coils cooled by a cryogenic medium for generating a homogeneous magnetic background field, with normal-conducting coils within the interior bounded by the background field coils for forming magnetic field gradients as well as with at least one cooled radiation shield of electrically and thermally conducting material which is arranged between the normal-conducting gradient coils and the superconducting background field coils. A magnetic structural arrangement of this nature is indicated in EP-A-0 144 171.
In the field of medical technology, imaging diagnostic procedures have been developed in which integral resonance signals of nuclei of a given chemical element of a 17 body, in particular a human body, or parts of the body are analyzed by calculations and measurements. On the basis of the thus to be obtained spatial spin density and/or relaxation time distribution, an image similar to the X-ray tomograph of computer tomography can be reconstructed or calculated. Corresponding methods are known generally under the term nuclear magnetic resonance tomography or magnetic resonance imaging or spectroscopy.
A precondition for nuclear magnetic resonance tomography is a magnetic field generated by a so-called background field magnet into which a body or body part to be examined is introduced along an axis which in general aligns with the axis of orientation of the magnetic background field. The background field must be sufficiently homogeneous in a corresponding imaging or examination area and its magnetic induction in these regions may be up to several Tesla. Such high magnetic inductions can, however, cost-effectively only be generated with superconducting magnetic coils which must be placed in a corresponding cryo-system. Such a cryo-system also comprises at least one cooled radiation shield in order to limit heat introduction of room temperature onto the superconducting background field coils. Corresponding radiation shields are, therefore, manufactured in general of thermally and, hence, also of electrically high-conducting material. The magnetic background field is superimposed by stationary and/or pulsed so-called gradient fields which are created by normal-conducting coils which are arranged within the inner space bounded by the cryo-system of the background field coils and are, in general, at approximately room temperature. For excitation with respect to a precession 17 motion of the individual atomic nuclei in the body or the body part, further, a special antenna device is required with which briefly a high-frequency magnetic alternating field can be generated. This antenna device can potentially also be used to receive the high-frequency signals produced by the thus excited atomic nuclei.
According to EP-A-0 144 171, the radiation shield of a superconducting background field coil system or its cryo-system is made of an electrically high-conducting material such as, for example, aluminum. In such a radiation shield, however, eddy currents are induced by the normal-conducting gradient coils which, in turn, regenerate gradient fields in the effective volume of the examination region.
Without special countermeasures, however, this brings about lack of sharpness of the images to be obtained. In this connection in radiation shields of known magnetic structural arrangements it is, in general, also not practicable to fabricate these so as to be non-shielding, for example, by using electrically low-conducting materials or with slits. In that case, the gradient fields can penetrate into the helium-cold region of the superconducting background field coils and there generate quantities of heat which are, given the very low operating temperature, particularly disturbing.
In connection with this set of problems, two countermeasures are primarily known which, however, are relatively elaborate:
(1) The magnetic field of the eddy current induced in a corresponding radiation shield is approximately 10 to 30% of the direct gradient field. The field connected to it is opposite to the original one, i.e. it thus attenuates it. The effect can be taken into consideration when laying-out and designing the gradient coils (cf. for example "J. Phys. E.", Vol. 19, 1986, pages 876 to 879 or EP-A-0 164 199).
(2) It is further known, for example from "J. Phys. D.", Vol. 19, 1986, pages L129 to L131 to introduce an additional system of gradient coils which is disposed between the actual primary gradient coil system and the radiation shield as close as possible to this shield. The limited space in the interior in this arrangement in the generally solenoidal superconducting background field leads to difficulties.